Sunday, 21st April 2024: While working with different HPC performance tools, Linux perf has always intrigued me. Over the years I have curiously watched Brendan Gregg's talks and have learned a lot from various examples. As a performance engineer, I find Linux perf's capabilities fascinating. However, I have struggled to fully engage with Linux perf. This disconnect can be attributed to various factors. For instance, perf is often unavailable or lacks necessary permissions on HPC systems, its focus is primarily on single-server analysis rather than distributed applications executing on a large number of servers, etc.… Read the rest
Navigating the Complexity of Large Codebases Using Vtune + xdot (or perf + gprof2dot)
Sunday, 7th April 2024: Back in September 2013, when I started my journey at the Blue Brain, I was navigating relatively large codebases for the first time. I was eager to gain a comprehensive understanding of code structures, their execution workflow, and performance aspects. During this period I started using Intel Vtune with xdot/gprof2dot and found it extremely useful. With this combination, I could generate detailed execution call graphs of applications and then sit together to deep dive into both the structural and performance aspects of the code with the senior engineers.… Read the rest
Understanding CPU Architecture And Performance Using LIKWID
March 2020: I was planning to write about CPU microarchitecture analysis for a long time. I started writing this post more than a year ago, just before the beginning of COVID-19. But with so many things happening around (and new parenting responsibilities 👧), this got delayed for quite a long time. Finally getting some weekend time to get this out!
Like previous blog posts, this also became longer and longer as I started writing details.… Read the rest
C-Reduce: Systematically Tackling (Not Only) Compiler Bugs
Sunday, 14th January 2024: Happy to get this post out within the first month of 2024! I started writing this in November 2023 and was hoping to get this out sooner. But then my second daughter 👧 arrived, putting some extra time constraints on how much free time I could find. Now, as I finally getting back to the schedule, I am delighted to bring this post to completion.
This blog post deviates a bit from the usual focus on performance-related aspects, and there's a specific reason for this.… Read the rest
pahole: Analysing Memory Layout of Complex Data Structures With Ease
Sunday, 5th November 2023: Putting together this blog post feels like a positive stride! As I mentioned in the previous post, (core-to-core latency tool), I'm aiming to integrate more consistent writing into my routine. While it took a month to pen this down, it's progress from the previous year 😇. Hoping that the upward trend will continue...!
During the past few weekends, I've been reading about the capabilities of the perf C2C (Cache To Cache) tool.… Read the rest
core-to-core-latency: A Nice Little Tool!
Saturday, 23rd Sept 2023: I've been curiously staring at my blog for quite some time, and it reminds me over and over again that it's been nearly two years since I managed to write new content here 😞. I have a few work-in-progress articles, and unfortunately, they've remained incomplete for quite some time. It's been a bit challenging to find dedicated long weekend hours to write the detailed posts that I really love.… Read the rest
LinkTest : Measuring Communication Latency and Bandwidth At Scale
October 2020: What makes supercomputers special? They have state-of-the-art processors, fast parallel file systems, specialized power & cooling infrastructure and complex software stack to run. But, a high-speed interconnect that tightly integrates thousands of nodes differentiate a supercomputer from a commodity cluster. Data movement within a node or across nodes is an important aspect for many scientific applications and hence low latency, high bandwidth interconnect technology is one of the key elements of the HPC systems.
Setting up such a system with tens of thousands of nodes and performance tuning is not an easy task. Especially during the early days of deployment and acceptance benchmarking where we often have to run various tests for weeks to identify issues, fix them and reach expected performance.… Read the rest
I/O Performance Analysis with Darshan
When optimizing parallel applications at scale, we often focus on computation-communication aspects and I/O often gets limited attention. With increasing performance gap between compute and I/O subsystems, improving I/O performance remains one of the major challenge. As filesystem is a shared resource, few jobs running on a system can significantly impact performance of other applications. In such scenario, even if we use profiling tool (see list here) to identify slow I/O routines, it's difficult to understand real cause. For example, there might be other applications dominating filesystem resulting in poor I/O performance.… Read the rest
Intel’s One API : What We Know and How to Get Ready
What is One API? This has been a common question since Intel announced One API vision during Intel Architecture Day back in December 2018. The aim of this is to deliver uniform software experience with optimal performance across broad range of Intel hardware. There has been some press releases and high level presentations depicting how One API is going to solve programming challenge for scalar processors (CPUs), vector processors (GPUs), matrix processors (AI accelerators) as well as spatial processing elements (FPGAs). For people waiting for Intel Xe as Xeon Phi successor, this is exciting.… Read the rest
First Screencast : Summary of Computing Laws!
Almost a decade ago I was involved in number of technical training activities, and I enjoyed it lot. Actually, I wanted to be a trainer before moving to academia/research. Let's keep that for later discussion 🙂 .
I didn't think about creating online videos (aka screencasting). Recently I came across Screenflow and saw how easy it is to create reasonably good quality videos/tutorials. During this weekend I decided to give it a try, and this is my first attempt!
Nothing fancy in this first screencast attempt: just took my last post and converted that to screencast. Learnt a lot of things about Screenflow.… Read the rest
Summary of Computing Laws : Amdahl, Dennard, Gustafson, Little, Moore and More…!
During one of the recent computing conference I heard the author saying:
With all the laws of computing fighting against us, the industry needs open interfaces to allow innovations...
I was curious about who all are fighting against us 🙂 . In this post I tried to put together summary of different laws related to computing (especially computing hardware trend, parallel performance and efficiency) that I came across over the years. Lets get started...!
Update If you prefer watching video instead of reading, you can look at my first screencast attempt here : First Screencast : Summary Of Computing Laws!… Read the rest
Blade : Cube’s OTF2 Trace Visualizer
My first experience with the Vampir trace visualiser was in 2010 during my studies at EPCC. While working on the exercises and samples, I was excited by the possibility of finding out what every process or thread (from thousands) is doing at any point in time. Over the years I have used TAU + Score-P + Vampir toolset with different applications on various systems. When it comes to trace visualisation for scientific applications at scale, Vampir is very impressive. If you haven't used it before, give a try!
One of the missing piece in profiling toolset (in my opinion) is an open source alternative for OTF2 trace visualisation.… Read the rest
Summary Of Python Profiling Tools – Part I
If you are working in the area of scientific computing, in academia or industry, most likely you are using Python in some form. Traditionally Python is described as slow when it comes to performance and there are number of discussions about speed compared to native C/C++ applications 1 2. The goal of this post is not to argue about performance but to summarise various tools that can help to find out performance bottlenecks before coming to such conclusions. In the previous post, I summarised more than 90 profiling tools that can be used for analysing performance of C/C++/Fortran applications.… Read the rest
Python Profiling : Deterministic vs Statistical Profilers
Different python profiling tools use different methodologies for gathering performance data and hence have different runtime overhead. Before choosing a profiler tool it is helpful to understand two commonly employed techniques for collecting performance data :
- Deterministic profiling Deterministic profilers execute trace functions at various points of interest (e.g. function call, function return) and record precise timings of these events. Typically this requires source code instrumentation but python provides hooks (optional callbacks) which can be used to insert trace functions.
- Statistical profiling Instead of tracking every event (e.g. call to every function), statistical profilers interrupt application periodically and collect samples of the execution state (call stack snapshots).
Summary of Debugging Tools for Parallel Applications
Nowadays it's not uncommon to run parallel applications with hundreds of thousands of processes on supercomputing platforms. Debugging these parallel applications with sporadic crashes, deadlocks, memory errors or incorrect results is a challenging task. There are number of tools available that help identifying and fixing bugs but one needs to understand tools, their capabilities and when they can be used. This post tries to summarise various debugging tools (open source as well as commercial).
Note that not all tools can be used with distributed applications. For example, open source tools like GDB and Valgrind are commonly used for debugging serial, multi-threaded applications.… Read the rest
Summary of Profiling Tools for Parallel Applications
Many scientific/industrial applications run on workstation to largest supercomputers in the world. With the continuous evolution of hardware platforms, achieving good performance is a challenging task. There are many profiling tools available to analyse and optimise the performance. But not all tools/methods are available on every platform, especially in high performance computing. First step in performance engineering workflow is to understand which tools are available and when they can be used. There is no one-size-fits-all solution : some are designed with broad feature list for high level analysis and others for specific platform with low level hardware metrics.
While choosing profiling tool one need to consider different aspects:
- Goal : Are you interested in high level performance metrics?